1. Field of the Invention
The present invention relates in general to devices and methods for estimating the concentration of ionic substances in water, and more particularly, to devices and methods for estimating, with an aid of a flow-injection analyzing method, the concentration of ammonium ions (NH.sub.4.sup.+), nitric acid ions (NO.sub.3.sup.-) and nitrous acid ions (NO.sub.2.sup.-) which are the three nitrogen-containing ionic substances in water.
2. Description of the Prior Art
For estimating the concentration of the above-mentioned three nitrogen-including ionic substances in water, such as water of rivers, lakes and marshes, various methods have been proposed and put into practical use, which are for example, ion exchange chromatography method, colorimetric method, neutralization titration method and ion electrode method.
As is known, the ion exchange chromatography method is one of high speed liquid chromatography methods using an ion exchange column, which has been developed particularly for analyzing inorganic anions and cations. That is, the ion exchange chromatography method can precisely estimate the inorganic anions, such as F.sup.-, Cl.sup.-, Br.sup.-, NO.sub.2.sup.-, NO.sub.3.sup.-, SO.sub.3.sup.2-, SO.sub.4.sup.2- and PO.sub.4.sup.2-. In this method, a column filled with anion exchange resin grains is used and a sample solution is led into the column from its top. With this, anions in the sample solution are adsorbed by the grains. Then, eluate containing a very small amount of competitive anion is led into the column. The amount of the competitive anion is so small that a conductivity meter can not detect it. Each anion adsorbed by the grains competes with the competitive anion and is eluted from the column with a certain mobility, so that the concentration of anion in the eluate can be estimated.
According to this ion exchange chromatography method, the ammonium ion can be estimated to a level of several ppm to several tens of ppm by using the conductivity meter. For the measurement, about ten minutes are usually needed from the time when the sample solution is led into the column. The quantitative concentration range of this method is relatively high, that is, from about 0.1 mg/l to about 30 mg/l.
Indophenol blue absorptiometric method is a typical one of the colorimetric methods. In this method, the indophenol blue which is produced when ammonium ion, with coexistence of hypochlorous acid ion, reacts with phenol is subjected to an absorbance test in which the absorbance of the light of 630 nm (nanometer) is measured. The quantitative concentration range of this method is relatively high, that is, from about 1.6 mg/l to about 33 mg/l.
In the neutralization titration method, ammonia, which has been extracted by effecting a distillation, is absorbed by a given amount of sulfuric acid (viz., 25 m mol/l) to prepare a sample solution, and the sample solution is subjected to a titration test using 50 m mol/l sodium hydroxide solution to estimate the ammonium ion. The quantitative concentration range of this method is relatively high, that is, from about 0.3 mg/l to about 40 mg/l.
In the ion electrode method, a sample, which has been subjected to a pretreatment, is added with sodium hydroxide solution to prepare a sample solution whose pH ranges from about 11 to about 13. With this process, ammonium ions are transformed to ammonia. By using an indicator electrode (viz., ammonia electrode), the potential of the sample solution to estimate the ammonium ions. The quantitative concentration range of this method is relatively high, that is, from about 0.1 mg/l to 100 mg/l.
As the colorimetric method, a so-called "sulfanilamide.naphthylethyldiamine method" and a so-called "phenoldisulfonic acid method" are also used. In the sulfanilamide.naphthylethyldiamine method, under acidity, sulfanilamide is led into water having nitrous acid ion. With this, the water becomes colored violet due to production of azo-coloring matter in the water. The colored water is then subjected to an absorptiometry to measure the absorbance of the color. With this, the concentration of nitrite nitrogen (NO.sub.2.sup.- --N) is determined. In the phenoldisulfonic acid method, sulfate is treated with phenoldisulfonic acid to produce nitrophenoldisulfornic acid solution which is colored yellow. The colored solution is then subjected to an absorptiometry to measure of the absorbance of the color. With this, the concentration of nitrate nitrogen (NO.sub.3.sup.- --N) is determined.
However, the above-mentioned methods have various drawbacks which are as follows.
That is, in case of the ion exchange chromatography method, a relative long measuring time (above ten minutes) is needed. Furthermore, as a pretreatment, filtering of the sample solution is needed for removing suspended solid and organic substances from the solution. Furthermore, water of rivers, lakes and marshes and sewage water can not be tested continuously because speedy treatment can not be made against contamination of them.
Usage of two types of ion exchange columns may be thought out. That is, one column is filled with cation exchange resin grains and the other column is filled with anion grains, and by switching the fluid passages to these two columns, cations and anions in a sample solution are estimated at the same time. However, this method brings about increase in number of movable portions of the equipment and thus increases the possibility of trouble. Furthermore, replacement of filters has to be made frequently for cleaning the sample solution.
In case of the colorimetric method, many manual labor operations are needed due to its inherence. Furthermore, a relatively large amount (viz., about 100 ml) of sample solution (viz., test water) is needed and about30 to 60 minutes are needed for the measurement. Thus, automization of this method is very difficult. Furthermore, due to the inherence, this method is can not be used for measuring the concentration in ppb level.
In case of the neutralization titration method and the ion electrode method, troublesome operations are needed and relatively long time is needed for the measurement. Furthermore, the quantitative concentration range of these methods are relatively high, and thus, satisfied measurement has not been obtained in measuring efficiency and measuring accuracy.